1. Field of the Invention
This invention relates to blood collection and more particularly relates to an additive for a collection tube which enhances clotting.
2. Background
Blood samples are routinely taken in evacuated tubes, such as glass VACUTAINER.TM. tubes (Becton, Dickinson and Company). One end of a double-ended needle is inserted into a patient's vein. The other end of the needle then punctures a septum covering the open end of the VACUTAINER.TM., tube so that the vacuum in the tube draws the blood sample through the needle into the tube. Using this technique, a plurality of samples can be taken using a single needle puncture of the skin. Plastic tubes have also been proposed for blood collection. Plastic offers a number of advantages such as lower breakage than glass tubes, less weight in shipment, and easier disposal by incineration.
Blood collected in evacuated tubes often must be clotted prior to clinical examination. It is desirable to form a dense clot as rapidly and completely as possible to facilitate clean separation of the clot from the serum layer by centrifugation. To achieve this end, both plastic and glass blood collection tubes frequently employ a clot activator.
Two types of activators, classified in the art according to the portion of the blood coagulation cascade stimulated, are conventionally employed. Particulate activators share a common biochemical mechanism of clot activation known as contact activation of the intrinsic pathway. Whole blood contains all of the necessary factors to cause clotting by the intrinsic pathway. Clot activation by the intrinsic pathway is surface area dependent, i.e., the time required to form a complete blood clot is dependent on the total number of activating surface sites per unit-area on the activator surface relative to the volume of blood. Greater surface area, provided by finely divided particulate activators, leads to shorter clot times. Particulate activators are used in practically all commercial blood collection tubes and lead to dense, crosslinked clots that cleanly separate from the serum in a hematological centrifuge. Clot formation, however, is relatively slow, and about 30-60 minutes are required prior to centrifugation. Typical particulate activators used commercially are silica impregnated in fabric, silica particles in small plastic cups or silicate particles applied to the tube wall in polyvinylpyrrolidone (PVP). When blood enters a tube containing silicate-PVP, the PVP dissolves, enters the serum and the silicate particles are released.
The second type of clot activators induces clotting through a different part of the coagulation cascade known in the art as the extrinsic pathway. The extrinsic system relies on the presence of a substance not normally present in whole blood. Activation is biochemical in nature and is concentration dependent. Clot activation rates are very high, leading to clot formation in 10-20 minutes, but clots resulting from the extrinsic pathway are gelatinous in nature and do not cleanly separate from serum. With extrinsic pathway activators, serum quality is frequently poor and may not meet the needs of sensitive clinical analysis. Further, contamination of serum by externally added blood-soluble protein activators is undesirable.
There is a need in the art for a clot activating additive for a blood collection tube which rapidly provides a dense clot which separates clearly from the serum without contaminating the serum with soluble chemicals which may interfere with blood analyses. The present invention is directed to fulfilling this need.